updated 11:11 pm EDT, Thu June 13, 2013

At present rate, Apple will hit 1B customers by 2015

A new chart by Horace Dediu at Asymco shows that, based on Apple's public pronouncements on iTunes accountholder growth, the company is adding approximately 500,000 new accounts each and every day. Apple CEO Tim Cook, during the WWDC keynote on Monday, said that Apple now had a total of 575 million iTunes account holders, most of whom have a credit card attached to the account. Given that rate of growth, Apple should expect to top 600 million well before the end of the year, and could see one billion account holders by mid-2015.

This rate would appear to be far ahead of any competitors, and represents a strong acceleration of growth beginning in 2012, when the number of iTunes customers roughly doubled from just over 200 million to 400 million between mid-2011 and mid-2012. In the nine months since then, Apple has added 175 million more users, according to Cook.

Assuming the numbers are accurate, a good comparison would be Amazon, which has said it has 200M "active accounts" during the last fiscal year. Although a few other firms, such as Google and Facebook, have many more users than Apple in total, none other than Amazon have a unified storefront with financial information attached to user accounts in the same way as iTunes, the accounts for which also work with the Mac App Store, iTunes Match and the iBookstore.

Apple recently revealed as part of its defense against the Department of Justice that it holds about 20 percent of the e-book market, about double what pundits had previously believed. The company said it had around 100 million iBookstore customers, though there is a lot of crossover between such buyers and App Store or iTunes buyers. The head of the iBookstore testified that Apple had doubled its e-book business in 2012, and that it had maintained roughly a 20 percent share since its inception in 2010.

The company's growth in iTunes accounts over the last two years reflect Apple's enormous mainstream reach with the release of the most recent iPhone, iPad, iPod touch and Mac lines, all of which request users set up an Apple ID which doubles as an iTunes account by default. While the attachment of a credit card isn't required, most users find it convenient as they can use it to instantly pay for apps, e-books, media purchases and more using a wide variety of mobile devices, Macs or Apple TV.